Identification By Double Refraction & Pleochroism

Pleochroism

When a beam of white light enters a colored doubly-refractive gem
material, either synthetic or natural, each of the beams, in addition
to traveling at different velocities, may suffer a characteristic absorption
of certain of its component wavelengths and emerge as a different color;
this is called PLEOCHROISM (from the Greek "pleo,"
meaning MORE, and "chroma" meaning COLOR). Regardless of the direction
observed, with the exception of the one direction or angle refraction,
stones of the tetragonal and hexagonal systems will display only two
pleochroic (pronounced PLEE-oh-KRO-ik) colors and are said to exhibit
dichroism(pronounced DYE-ho-izm) or to be DICHROIC
(pronounced dye-KRO-ik). Stones of the orthorhombic, monoclinic and
triclinic systems may show a total of three distinct pleochroic colors,
but only two will be visible from ANY ONE DIRECTION; these gems are
said to exhibit TRICHROISM (pronounced TRY-kro-izm)
or to be TRICHROlC (pronounced try-KRO-ik). Stones of the tetragonal
and hexagonal systems will never show more than two colors, but the
remaining systems may show a total of two or three colors, for in some
gems there is no discernable difference in color in two of the three
directions.

Tests for Double Refraction

Several methods may be employed in order to determine whether a transparent
gem possesses double refraction or not. One of the simplest is to look
through the table of a stone with a loupe and observe whether the edges
between the back facets appear as single or double images. Through practice,
one may become sufficiently proficient to distinguish the difference
between ruby, which has a low birefringence, and red spinel and red
garnet, both of which are singly refractive. This is also an easy way
to distinguish colorless zircon or synthetic rutile, which are doubly
refractive, from diamond, which is singly refractive. Since glass is
singly refractive, the test may also sometimes be helpful in distinguishing
between genuine doubly-refractive stones and glass.

Because one might look along one of the two possible directions of
single retraction in a doubly refractive stone, the stone should be
examined in at least three directions. Examination in additional directions
may also assist, because the doubling of the edges increases from none,
parallel to the optic axis (the direction of single refraction), to
a maximum at right angles to the optic axis. Also, the distance between
the twin images of the facet edges increases with an increase in the
depth of the stone. When first seen by a student, the doubled facet
edges are often likened to the appearance of railroad tracks.

THE POLARISCOPE (pronounced po-LARE-uh-scope) is an inexpensive and
very satisfactory instrument for the detection of double refraction.
It employs Polaroid (trademark) plates to accomplish the necessary polarization.
Polaroid is made by bonding on plastic film a very strongly dichroic
material composed of tiny crystals in parallel positions, optically.
Thus they behave as a single large crystal plate, the dichroism of which
is so strong that light in one of the two vibration directions is almost
totally absorbed, whereas the other direction pusses light readily.

The polarizer's, which are attached to the instrument, are rotated
with respect to one another until little or not light is allowed to
pass, The gem is then placed between the polarizer. If the gem is doubly
refractive and is not being observed parallel to an optic axis as it
is rotated, it will appear dark at every 90° position and light at the
intermediate positions. Amorphous materials and gems of the cubic system
are not doubly refractive and do not affect light in this manner; they
remain dark in ALL positions when viewed between the polarizer. In order
to be sure that observations are correct, it is necessary to turn the
gem through several positions.

Some amorphous materials and gems of the cubic system show ANOMALOUS
(pronounced ah-NAHM-ah-Iuss), or false, double refraction ( which is
caused by internal strain. Practice with the use of the polariscope
soon enables one to distinguish between anomalous double refraction
and true double refraction.

Tests for Pleochroism

Pleochroism is most easily observed with a DICHROSCOPE (pronounced
DYE-kro-scope), a small instrument that is capable of separating the
two polarized beams so that they can been seen separately. Looking through
a dichroscope, one sees twin images of the small opening in the opposite
end of the instrument. These double images are produced by a piece of
optical quality calcite when a dichroic stone is viewed, the two squares
may appear different colors. Since the two squares appear side by side,
it is possible to observe even weak dichroism. The colors of trichroic
gems may be observed by first viewing one pair of colors and then shifting
the gem with respect to the dichroscope and viewing a second pair, one
of which is repeated from the first. It is necessary to observe a gem
along at least three directions before deciding it is not pleochroic,
in order to be sure that the first two observations were not made parallel
to the optic axes of a biaxial stone. Both the strength of the pleochroism
observed and the colors seen are taken into consideration in determinations.
Degrees of pleochroism are expressed as very strong, strong, distinct,
weak, and very weak.

Pleochroism may also be observed with the polariscope, by turning
the upper Polaroid so that maximum light is transmitted and rotating
the gem between the polarizer. If it is a pleochroic gem and is not
being observed parallel to an optic axis, it will change color during
the rotation. As a rule, weak pleochroism is more difficult to observe
with the polariscope than with the dichroscope, but even fairly weak
color differences are evident to an observer who has had sufficient
practice in using a polariscope.

The primary value of the observation of pleochroism is that its presence
is PROOF OF DOUBLE REFRACTION: The polariscope detects
doubly-refractive gems even if they are colorless or very pale in color;
this determination is not possible with the dichroscope. However, the
dichroscope does show the exact NATURE of the pleochroic colors, an
observation that is sometimes of value in identification.

Moreover, the dichroscope does not show pleochroism in those ordinarily
singly-refractive colored materials that exhibit anomalous double refraction.
As stated previously, experience permits the observer to distinguish
between true and anomalous double refraction in the polariscope, but
this distinction is sometimes difficult. Generally speaking, the polariscope
is a more valuable testing instrument than the dichroscope; for best
results in gem testing, however, it is advisable that the two be used
in conjunction with one another.

In his early observations with the dichroscope, the student gemologist
usually expects to see a great difference between the pleochroic colors.
New students often decide that gems exhibiting weak or even distinct
pleochroic colors have no pleochroism whatever, because the difference
between the colors is not obvious. The ability to detect weak and very
weak pleochroism increases with practice, as does the ability to see
slight differences of color in apparently colorless diamonds. Often
one color will be simply a trifle lighter or darker or of a slightly
different hue.